One method for manufacturing optical fibers comprises depositing multiple thin films or layers of glass (e.g., glass layers) on the inside surface of a hollow substrate tube. Subsequently, the substrate tube is collapsed to form a core-rod, which is optionally sleeved or overcladded to form an optical-fiber preform from which optical fibers may be drawn.
The substrate tube has an outer surface (i.e., the outer surface of the wall of the substrate tube) and an inner surface (i.e., the inner surface of the wall of the substrate tube). The inner surface of the substrate tube is in contact with a cavity present on the inside of the substrate tube. In an embodiment, the substrate tube is cylindrical in shape and hence provides (or encloses) a cylindrical cavity.
The glass layers are applied on the inside of the substrate tube by introducing glass-forming gases (e.g., doped or undoped reactive gases) into the interior of the substrate tube from one end (i.e., the supply side of the substrate tube). Doped or undoped glass layers are deposited onto the interior surface of the substrate tube. The gases are discharged or removed from the other end of the substrate tube (i.e., the discharge side of the substrate tube), optionally by a vacuum pump. A vacuum pump generates a reduced pressure within the interior of the substrate tube.
During a PCVD (plasma chemical vapor deposition) process, a localized plasma is generated. Generally, electromagnetic radiation is directed toward an applicator via a waveguide. The applicator, which surrounds a glass substrate tube, couples the radiation into the plasma. In addition, the applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900-1300° C. during the deposition process. The applicator (and hence the plasma it forms) is moved reciprocally in the substrate tube's longitudinal direction. A thin glass layer is deposited onto the interior surface of the substrate tube with every stroke or pass of the applicator.
The applicator is moved in translation over the length of the substrate tube within the boundaries of a surrounding furnace. With this translational movement of the applicator, the plasma also moves in the same direction. As the applicator reaches the furnace's inner wall near one end of the substrate tube, the movement of the applicator is reversed (this point is a reversal point) so that it moves to the other end of the substrate tube toward the furnace's other inner wall (and another reversal point). The applicator and thus the plasma travel in a back-and-forth movement along the length of the substrate tube. Each reciprocating movement is called a “pass” or a “stroke.” Going from the reversal point near the supply side to the reversal point near the discharge side is a forward stroke or pass. Going from the reversal point near the discharge side to the reversal point near the supply side is a backward stroke or pass. With each pass, a thin layer of glass is deposited on the substrate tube's inside surface.
Normally, a plasma is generated only in a part of the substrate tube (e.g., the part surrounded by the microwave applicator), which part is called a plasma zone. Typically, the dimensions of the microwave applicator are smaller than the respective dimensions of the furnace and the substrate tube. Only at the position of the plasma are the reactive gases converted into glass and deposited on the inside surface of the substrate tube.
The passes increase the cumulative thickness of these thin films (i.e., the deposited material), which decreases the remaining internal diameter of the substrate tube. In other words, the hollow space inside the substrate tube gets progressively smaller with each pass.
This plasma causes the reaction of the glass-forming gases (e.g., O2, SiCl4, and, for instance, dopant gas GeCl2 or other gases) that are supplied to the inside of the substrate tube. The reaction of the glass-forming gases allows reaction of Si (silicon), O (oxygen), and, for instance, the dopant Ge (germanium) to effect direct deposition of, for example, Ge-doped SiOx on the inner surface of the substrate tube. A substrate tube having a plurality of vitrified glass layers within is called a deposited tube (with a surrounding substrate tube). In an embodiment, the substrate tube is removed from the plurality of vitrified glass layers. This remaining tube consisting merely of deposited glass layers is also called a deposited tube.
When the deposition is complete, the deposited tube (with or without surrounding substrate tube) is heated to close the central cavity (“collapsed”) to obtain a massive solid rod. This can optionally be externally provided with additional glass to increase its outer diameter, such as by applying silica by means of an outside deposition process or by placing the solid rod in a so-called jacket tube (or sleeve)—comprised of undoped silica—prior to the optical fiber drawing procedure, so as to increase the capacity of the optical-fiber preform thus obtained.
If an extremity of the optical-fiber preform is heated so that it becomes molten, a thin glass fiber can be drawn from the rod and be wound on a reel; the optical fiber then has a core portion and a cladding portion with relative dimensions and refractive indexes corresponding to those of the optical-fiber preform. The optical fiber can function as a waveguide, for example, for propagating optical telecommunication signals.
The glass-forming gases flushed through the substrate tube may also contain other components. The addition of a dopant gas such as C2F6 to glass-forming gases will lead to a reduction in the refractive index value of the silica.
Using an optical fiber for telecommunication requires that the optical fiber be substantially free from defects (e.g., discrepancies in the percentage of dopants, undesirable cross-sectional ellipticity, and the like), because, when considered over a large length of the optical fiber, such defects may cause a significant attenuation of the signal being transmitted. It is important, therefore, to realize a uniform and reproducible deposition process, because the quality of the deposited layers will eventually determine the quality of the fibers.
In order to have a good initial glass layer attachment on the interior wall of the substrate tube and to prevent the formation of bubbles in these initial glass deposited layers, preform manufacturers must pre-treat the inside of the substrate tube before the deposition process starts. This is also called a plasma-polishing or plasma-etching phase. Hence, generally before starting the deposition of glass layers inside the substrate tube, the inner surface of the initial substrate tube is pretreated or activated for achieving good adhesion and/or for preventing unwanted effects from pollutions that are present in the starting glass material of the substrate tube. This pretreatment or activation is generally carried out by etching. This etching is generally carried out by reciprocating a plasma in the substrate tube while flowing an etching gas—comprising a fluorine-containing compound, for example, FREON (C2F6), and optionally a carrier gas, such as oxygen (O2)—through the substrate tube. Such a treatment will etch away glass material from the inside of the substrate tube. Usually after this treatment, the substrate tube has lost around 5-50 grams of silica from the inside surface.
This plasma polishing has been found to cause preferential etching on the inside surface of the substrate tube that can also vary between substrate tubes (e.g., batches). The preferential etching gives local disturbances in the vitrified silica layers deposited in the subsequent CVD process.
This means that on a small scale, one encounters big differences in the amount of material being etched away, which can cause increase the roughness of the inside surface. Due to this phenomenon, when the amount of deposited material increases, this initial roughness will create disturbances in the end product. This is especially severe for a multimode optical-fiber product as the refractive index profile will be also modified, thereby causing a degradation of quality.
Especially when many layers are deposited, the initial surface irregularities are amplified so clear distortions are visible in the resulting core rod. These irregularities may degrade the optical fiber drawn therefrom. Additionally, the plasma polishing may lead to many small (<<1 millimeter) distortions in the inside surface, especially in fluorine-doped tubes. These distortions will increase during the deposition process and finally droplet-type distortions will be evident on the inner surface after the deposition process. This is undesirable.
In order to prevent this uneven etching, in prior art methods, the substrate tube is usually washed before the employment in the lathe using hydrofluoric acid. The HF acid, because it flows through the tube, can improve the surface in such a way that the effects of the etching in the plasma-polishing phase are less severe for the surface. Unfortunately, HF is a highly hazardous material, which makes the use of it very risky from the environment and safety viewpoint. Thus, another solution has to be provided to overcome uneven etching.
The present inventors have previously devised a solution to this problem, which is disclosed in commonly assigned European Patent No. 2,743,237, which is hereby incorporated by reference in its entirety. This European patent discloses a procedure in which the inner surface of a glass substrate tube is activated by (i) first depositing a number of activation glass layers on the inner surface of the substrate tube having a total thickness of at least 10 micrometers and at most 250 micrometers, and (ii) second, at least partially (at least 30 percent) removing the activation glass layers by etching. Although this procedure is very effective, it takes additional time (typically 10 minutes) as well as additional glass-forming material. Moreover, this procedure is rather complex and may hence introduce errors and consequently a decreased yield. Furthermore, this procedure is not very suitable for fluorine-doped substrate tubes, which are softer, because it can lead to an increased, final non-circularity by using this filling and sanding procedure.